Archimedes’ Principle, Buoyancy, Spar Deck, Freeboard, Green Water, Bulkheads, Watertight Compartments, RMS Titanic and Edmund Fitzgerald

Posted by PITHOCRATES - January 2nd, 2013

Technology 101

(Originally published April 4th, 2012)

The Spar Deck or Weather Deck is Where you Make a Ship Watertight

Let’s do a little experiment.  Fill up your kitchen sink with some water.  (Or simply do this the next time you wash dishes).  Then get a plastic cup.  Force the cup down into the water with the open side up until it rests on the bottom of the sink.  Make sure you have a cup tall enough so the top of it is out of the water when resting on the bottom.  Now let go of the cup.  What happens?  It bobs up out of the water.  And tips over on its side.  Where water can enter the cup.  As it does it weighs down the bottom of the cup and lifts the open end out of the water.  And it floats.  Now repeat this experiment.  Only fill the plastic cup full of water.  What happens when you let go of it when it’s sitting on the bottom of the sink?  It remains sitting on the bottom of the sink.

What you’ve just demonstrated is Archimedes’ principle.  The law of buoyancy.  Which explains why things like ships float in water.  Even ships made out of steel.  And concrete.  The weight of a ship pressing down on the water creates a force pushing up on the ship.  And if the density of the ship is less than the density of the water it will float.  Where the density of the ship includes all the air within the hull.  Ships are buoyant because air is less dense than water.  If water enters the hull it will increase the density of the ship.  Making it heavier.  And less buoyant.  As water enters the hull the ship will settle lower in the water.

The spar deck or weather deck is where you make a ship watertight.  This is where the hatches are on cargo ships.  We call the distance between the surface of the water and the spar deck freeboard.  A light ship doesn’t displace much water and rides higher in the water.  That is, it has greater freeboard.  With less ship in the water there is less resistance to forward propulsion.  Allowing it to travel faster.  However, a ship riding high in the water is much more sensitive to wave action.  And more susceptible to rolling from side to side.  Increasing the chance of rolling all the way over in heavy seas.  (Interestingly, if the ship stays watertight it can still float capsized.)  So ship captains have to watch their freeboard carefully.  If the ship rides too high (like an empty cargo ship) the captain will fill ballast tanks with water to lower the ship in the water.  By decreasing freeboard the ship is less prone to wave action.  But by lowering the spar deck closer to the surface of the water bigger waves can crash over the spar deck.  Flooding the spar deck with ‘green water’.  Common in a storm with high winds creating tall waves.  As long as the spar deck is watertight the ship will stay afloat.  And the solid water that washes over the spar deck will run off the ship and back into the sea.

The Titanic and the Fitzgerald were Near Unsinkable Designs but both lost Buoyancy and Sank

Improvements in ship design have made ships safer.  Steel ships can take a lot of damage and still float.  Ships struck by torpedoes in World War II could still float even with a hole below their waterline thanks to watertight compartments.  Where bulkheads divide a ship’s hull.  Watertight walls that typically run up to the weather deck.  Access though these bulkheads is via watertight doors.  These are the doors that close when a ship begins to take on water and the captain orders, “Close watertight doors.”  This contains the water ingress to one compartment allowing the ship to remain buoyant.  If it pitches down at the bow or lists to either side they can offset this imbalance with their ballast tanks.  Emptying the tanks where the ship is taking on water.  And filling the tanks where it is not.  To level the ship and keep it seaworthy until it reaches a safe harbor to make repairs.

They considered RMS Titanic unsinkable because of these features.  But they didn’t stop her from sinking on a calm night in 1912.  Why?  Two reasons.  The first was the way she struck the iceberg.  She sideswiped the iceberg.  Which cut a gash below the waterline in five of her ‘watertight’ compartments.  Which basically removed the benefit of compartmentalization.  They could not isolate the water ingress to a single compartment.  Or two.  Or three.  Even four.  Which she might have survived and remained afloat.  But water rushing into five compartments was too much.  It pitched the bow down.  And as the bow sank water spilled over the ‘watertight’ bulkheads and began flooding the next compartment.  Even ones the iceberg didn’t slash open.  As water poured over these bulkheads and flooded compartment after compartment the bow sank deeper and deeper into the water.  Until the unsinkable sank.  The Titanic sank slowly enough to rescue everyone on the ship.  She just didn’t carry enough lifeboats.  For they thought she was unsinkable.  Because of this lack of lifeboats 1,517 died.  Of course, having enough lifeboats doesn’t guarantee everyone will survive a sinking ship.

The Edmund Fitzgerald was the biggest ore carrier on the Great Lakes during her heyday.  These ships could take an enormous amount of abuse as the storms on the Great Lakes could be treacherous.  Like the one that fell on the Fitzgerald one November night in 1975.  When 30-foot waves hammered her and her sister ship the Arthur Andersen.  No one knows for sure what happened that night but some of the clues indicate she may have bottomed out on an uncharted shoal.  For she lost her handrails indicating that the ship may have hogged (where the bow and stern bends down from the center of the ship held up by that uncharted shoal).  The handrails were steel cables under tension running around the spar deck.  If the ship hogged this would have stretched the cable until it snapped.  She had green water washing across her deck.  Lost both of her radars.  A vent.  Maybe even a hatch cover.  Whatever happened she was taking on water.  A lot of it.  More than her pumps could keep up with.  Causing a list.  And the bow to settle deeper in the water.  Waves crashed over her bow as well as the Andersen’s.  The ships disappeared under the water.  Then reemerged.  As they design ships to do.  Then two massive waves rocked the Andersen.  She was following the Fitzgerald to help her navigate by the Andersen’s radar.  So these two waves had hit the Fitzgerald first.  The Fitzgerald had by this time taken on so much water that she lost too much freeboard.  When she disappeared under these two waves she never came back up.  It happened so fast there was no distress call.  The ship was longer than the lake was deep.  So her screw was still propelling the ship forward when the bow stuck the bottom.  She had lifeboat capacity for all 29 aboard.  But the ship sank too fast to use them.  Or even for the Andersen to see her as she sailed over her as she came to a rest on the bottom.

Our Ships have never been Safer but Ship Owners and Merchants still need to Protect their Wealth with Marine Insurance

We build bigger and bigger ships.  And it’s amazing what can float considering how heavy these ships can be.  But thanks to Archimedes’ principle all we have to do to make the biggest and heaviest ships float is too keep them watertight.  Keeping them less dense than the water that makes them float.  Even if we fail here due to events beyond our control we can isolate the water rushing in by sealing watertight compartments.  And keep them afloat.  So our ships have never been safer.  In addition we have far more detailed charts.  And satellite navigation to carefully guide us to our destination.  Despite all of this ships still sink.  Proving the need for something that has changed little since 14th century Genoa.  Marine insurance.  Because accidents still happen.  And ship owners and merchants still need to protect their wealth.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

Corduroy Roads, Positive Buoyancy, Negative Buoyancy, Carbon Dioxide, Crush Depth, Pressurization, Rapid Decompression and Space

Posted by PITHOCRATES - May 9th, 2012

Technology 101

Early Submarines could not Stay Submerged for Long for the Carbon Dioxide the Crew Exhaled built up to Dangerous Levels

People can pretty much walk anywhere.  As long as the ground is fairly solid beneath our feet.  Ditto for horses.  Though they tend to sink a little deeper in the softer ground than people do.  Carts are another story.  And artillery trains.  For their narrow wheels and heavy weight distributed on them tend to sink when the earthen ground is wet.  Early armies needing to move cannon and wagons through swampy areas would first build roads through these areas.  Out of trees.  Called corduroy roads.  It was a bumpy ride.  But you could pull heavy loads with small footprints through otherwise impassable areas.  As armies mechanized trucks and jeeps with fatter rubber tires replaced the narrow wheels on wagons.  Then tracked vehicles came along.  Allowing the great weights of armored vehicles with large guns to move across open fields.  The long and wide footprints of these vehicles distributing that heavy weight over a larger area.  Still, nothing can beat the modern rubber tire on a paved road for a smooth ride.  And the lower resistance between tire and road increases gas mileage.  Which is why trucks like to use as few axles on their trailers as possible.  For the more tires on the road the more friction between truck and road.  And the higher fuel consumption to overcome that friction.  Which is why we have to weigh trucks for some try to cheat by pulling heavier loads with too few axles.  When they do the high weight distributed through too few wheels will cause great stresses on the roadway.  Causing them to break and crumble apart.   

Man and machine can move freely across pretty much anything.  If we don’t carry food and water with us we could even ‘live off the land’.  But one thing we can’t do is walk or drive on water.  We have to bridge streams and rivers.  Go around lakes.  Or move onto boats.  Which can drive on water.  If they are built right.  And are buoyant.  Because if a boat weighed less than the water it displaced it floated.  Much like a pair of light-weight, spongy flip-flops made out of foam rubber.  Throw a pair into the water and they will float.  Put them on your feet and step into the deep end of a pool and you’ll sink.  Because when worn on your feet the large weight of your body distributed to the light pair of flip-flops makes those flip-flops heavier than the water they displace.  And they, along with you, sink.  Unlike a boat.  Which is lighter than the water it displaces.  As long as it is not overloaded.  Even if it’s steel.  Or concrete.  You see, the weight of the boat includes all the air inside the hull.  So a large hull filled with cargo AND air will be lighter than the water it displaces.  Which is why boats float. 

Early sail ships had great range.  As long as the wind blew.  Their range only being limited by the amount of food and fresh water they carried.  Later steam engines and diesel-electric engines had greater freedom in navigation not having to depend on the prevailing winds.  But they had the same limitations of food and water.  And when we took boats under the water we had another limitation.  Fresh air.  Early submarines could not stay submerged for long.  For underwater they could not pull air into a diesel-electric engine.  So they had to run on batteries.  Which had a limited duration.  So early subs spent most of their time on the surface.  Where they could run their diesel engines to recharge their batteries.  And open their hatches to get fresh air into the boat.  For when submerged the carbon dioxide the crew exhaled built up.  If it built up too much you could become disoriented and pass out.  And die.  If a sub is under attack staying under water for too long and the levels of carbon dioxide build up to dangerous levels a captain has little choice but to surface and surrender.  So the crew can breathe again.

Rapid Decompression at Altitude can be Catastrophic and Violent

Being in a submarine has been historically one of the more dangerous places to be in any navy (second to being on the deck of an aircraft carrier).  Just breathing on a sub had been a challenge at times while trying to evade an enemy destroyer.  But there are other risks, too.  Some things float.  And some things sink.  A submarine is somewhere in between.  It will float on the surface when it has positive buoyancy.  And sink when it has negative buoyancy.  But submarines operate in the oceans.  Which are very deep.  And the deeper you go the greater the pressure of the water.  Because the deeper you go there is more ocean above you pressing down on you.  And oceans are heavy.  If a sub goes too deep this pressure will crush the steel hull like a beer can.  What we call crush depth.  Killing everyone on board.  So a sub cannot go too deep.  Which makes going below the surface a delicate and risky business.  To submerge they flood ballast tanks.  Replacing air within the hull with water.  Making it sink.  Other tanks fill with water as necessary to ‘trim’ the boat.  Make it level under water.  When under way they use forward propulsion to maintain depth and trim with control surfaces like on an airplane.  If everything goes well a submarine can sink.  Then stop at a depth below the surface.  And then resurface.  Modern nuclear submarines can make fresh water and clean air.  So they can stay submerged as long as they have food for the crew to eat.

An airplane has no such staying power like a sub.  For planes have nothing to keep them in air but forward propulsion.  So food and water are not as great an issue.  Fuel is.  And is the greatest limitation on a plane.  In the military they have special airplanes that fly on station to serve as gas stations in the air for fighters and bombers.  To extend their range.  And it is only fuel they take on.  For other than very long-range bombers a flight crew is rarely in the air for extended hours at a time.  Some bomber crews may be in the air for a day or more.  But there are few crew members.  So they can carry sufficient food and water for these longer missions.  As long as they can fly they are good.  And fairly comfortable.  Unlike the earlier bomber crews.  Who flew in unpressurized planes.  For it is very cold at high altitudes.  And there isn’t enough oxygen to breathe.  So these crew members had to wear Arctic gear to keep from freezing to death.  And breathe oxygen they carried with them in tanks.  Pressurizing aircraft removed these problems.  Which made being in a plane like being in a tall building on the ground.  Your ears may pop but that’s about all the discomfort you would feel.  If a plane lost its pressurization while flying, though, it got quite uncomfortable.  And dangerous. 

Rapid decompression at altitude can be catastrophic.  And violent.  The higher the altitude the lower the air pressure.  And the faster the air pressure inside the airplane equals the air pressure outside the airplane.  The air will get suck out so fast that it’ll take every last piece of dust with it.  And breathable air.  Oxygen masks will drop in the passenger compartment.  The flight attendants will scramble to make sure all passengers get on oxygen.  As does the flight crew.  Who call in an emergency.  And make an emergency descent to get below 10 thousand feet.  Almost free falling out of the sky while air traffic control clears all traffic from beneath them.  Once below 10 thousand feet they can level off and breathe normally.  But it will be very, very cold.

Man’s Desire is to Go where no Man has Gone before and where no Human Body should Be

Space flight shares some things in common with both submarines and airplanes.  Like airplanes they can’t fly without fuel.  The greatest distance we’ve ever flown in space was to the moon and back.  The Saturn V rocket of the Apollo program was mostly fuel.   The rocket was 354 feet tall.  And about 75% of it was a fuel tank.  In 3 stages.  The first stage burned for about 150 seconds.  The second stage burned for about 360 seconds.  The third stage burned for about 500 seconds (in two burns, the first to get into earth orbit and the second to escape earth orbit).  Add that up and it comes to approximately 16 minutes.  After that the astronauts were then coasting at about 25,000 miles per hour towards the moon.  Or where the moon would be when they get there.  The pull of earth’s gravity slowed it down until the pull of the moon’s gravity sped it back up.  So that’s a lot of fuel burned at one time to hurl the spacecraft towards the moon.  The remaining fuel on board used for minor course corrections.  And to escape lunar orbit.  For the coast back home.  There was no refueling available in space.  So if something went wrong there was a good chance that the spacecraft would just float forever through the universe with no way of returning home.  Much like a submarine that can’t keep from falling in the ocean.  If it falls too deep it, too, will be unable to return home.

Also like in a submarine food and fresh water are critical supplies.  They brought food with them.  And made their own water in space with fuel cells.  It had to last for the entire trip.  About 8 days.  For in space there were no ports or supply ships.  You were truly on your own.  And if something happened to your food and water supply you didn’t eat or drink.  If the failure was early in the mission you could abort and return home.  If you were already in lunar orbit it would make for a long trip home.  The lack of food and hydration placing greater stresses on the astronauts making the easiest of tasks difficult.  And the critical ones that got you through reentry nearly impossible.  Also like on a submarine fresh air to breathe is critical.  Even more so because of the smaller volume of the spacecraft.  Which can fill up with carbon dioxide very quickly.  And unlike a sub a spacecraft can’t open a hatch for fresh air.  All they can do is rely on a scrubber system to remove the carbon dioxide from their cramped quarters.

While a submarine has a thick hull to protect it from the crushing pressures of the ocean an airplane has a thin aluminum skin to keep a pressurized atmosphere inside the aircraft.  Just like a spacecraft.  But unlike an aircraft, a spacecraft can’t drop below 10,000 feet to a breathable atmosphere in the event of a catastrophic depressurization.  Worse, in the vacuum of space losing your breathable atmosphere is the least of your troubles.  The human body cannot function in a vacuum.  The gases in the lungs will expand in a vacuum and rupture the lungs.  Bubbles will enter the bloodstream.  Water will boil away (turn into a gas).  The mouth and eyes will dry out and lose their body heat through this evaporation.  The water in muscle and soft tissue will boil away, too.  Causing swelling.  And pain.  Dissolved nitrogen in the blood will reform into a gas.  Causing the bends.  And pain.  Anything exposed to the sun’s ultraviolet radiation will get a severe sunburn.  Causing pain.  You will be conscious at first.  Feeling all of this pain.  And you will know what is coming next.  Powerless to do anything about it.  Brain asphyxiation will then set in.  Hypoxia.  The body will be bloated, blue and unresponsive.  But the brain and heart would continue on.  Finally the blood boils.  And the heat stops.  In all about a minute and half to suffer and die.

Man is an adventurer.  From the first time we walked away from our home.  Rode the first horse.  Harnessed the power of steam.  Then conquered the third dimension in submarines, airplanes and spacecraft.  We are adventurers.  It’s why we crossed oceans and discovered the new world.  Why we climbed the highest mountains.  And descended to the oceans’ lowest depth.  Why we fly in airplanes.  And travelled to the moon and back.  When things worked well these were great adventures.  When they did not they were horrible nightmares.  While a few seek this adventure most of us are content to walk the surface of the earth.  To feel the sand through our toes.   Or walk to the poolside bar in our flip-flops.  To enjoy an adult beverage on a summer’s day.  While adventurers are still seeking out something new.  And waiting on technology to allow them to go where no man has gone before.  Especially if it’s a place no human body should be.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,

Archimedes’ Principle, Buoyancy, Spar Deck, Freeboard, Green Water, Bulkheads, Watertight Compartments, RMS Titanic and Edmund Fitzgerald

Posted by PITHOCRATES - April 4th, 2012

Technology 101

The Spar Deck or Weather Deck is Where you Make a Ship Watertight

Let’s do a little experiment.  Fill up your kitchen sink with some water.  (Or simply do this the next time you wash dishes).  Then get a plastic cup.  Force the cup down into the water with the open side up until it rests on the bottom of the sink.  Make sure you have a cup tall enough so the top of it is out of the water when resting on the bottom.  Now let go of the cup.  What happens?  It bobs up out of the water.  And tips over on its side.  Where water can enter the cup.  As it does it weighs down the bottom of the cup and lifts the open end out of the water.  And it floats.  Now repeat this experiment.  Only fill the plastic cup full of water.  What happens when you let go of it when it’s sitting on the bottom of the sink?  It remains sitting on the bottom of the sink.

What you’ve just demonstrated is Archimedes’ principle.  The law of buoyancy.  Which explains why things like ships float in water.  Even ships made out of steel.  And concrete.  The weight of a ship pressing down on the water creates a force pushing up on the ship.  And if the density of the ship is less than the density of the water it will float.  Where the density of the ship includes all the air within the hull.  Ships are buoyant because air is less dense than water.  If water enters the hull it will increase the density of the ship.  Making it heavier.  And less buoyant.  As water enters the hull the ship will settle lower in the water.

The spar deck or weather deck is where you make a ship watertight.  This is where the hatches are on cargo ships.  We call the distance between the surface of the water and the spar deck freeboard.  A light ship doesn’t displace much water and rides higher in the water.  That is, it has greater freeboard.  With less ship in the water there is less resistance to forward propulsion.  Allowing it to travel faster.  However, a ship riding high in the water is much more sensitive to wave action.  And more susceptible to rolling from side to side.  Increasing the chance of rolling all the way over in heavy seas.  (Interestingly, if the ship stays watertight it can still float capsized.)  So ship captains have to watch their freeboard carefully.  If the ship rides too high (like an empty cargo ship) the captain will fill ballast tanks with water to lower the ship in the water.  By decreasing freeboard the ship is less prone to wave action.  But by lowering the spar deck closer to the surface of the water bigger waves can crash over the spar deck.  Flooding the spar deck with ‘green water’.  Common in a storm with high winds creating tall waves.  As long as the spar deck is watertight the ship will stay afloat.  And the solid water that washes over the spar deck will run off the ship and back into the sea.

The Titanic and the Fitzgerald were Near Unsinkable Designs but both lost Buoyancy and Sank

Improvements in ship design have made ships safer.  Steel ships can take a lot of damage and still float.  Ships struck by torpedoes in World War II could still float even with a hole below their waterline thanks to watertight compartments.  Where bulkheads divide a ship’s hull.  Watertight walls that typically run up to the weather deck.  Access though these bulkheads is via watertight doors.  These are the doors that close when a ship begins to take on water and the captain orders, “Close watertight doors.”  This contains the water ingress to one compartment allowing the ship to remain buoyant.  If it pitches down at the bow or lists to either side they can offset this imbalance with their ballast tanks.  Emptying the tanks where the ship is taking on water.  And filling the tanks where it is not.  To level the ship and keep it seaworthy until it reaches a safe harbor to make repairs.

They considered RMS Titanic unsinkable because of these features.  But they didn’t stop her from sinking on a calm night in 1912.  Why?  Two reasons.  The first was the way she struck the iceberg.  She sideswiped the iceberg.  Which cut a gash below the waterline in five of her ‘watertight’ compartments.  Which basically removed the benefit of compartmentalization.  They could not isolate the water ingress to a single compartment.  Or two.  Or three.  Even four.  Which she might have survived and remained afloat.  But water rushing into five compartments was too much.  It pitched the bow down.  And as the bow sank water spilled over the ‘watertight’ bulkheads and began flooding the next compartment.  Even ones the iceberg didn’t slash open.  As water poured over these bulkheads and flooded compartment after compartment the bow sank deeper and deeper into the water.  Until the unsinkable sank.  The Titanic sank slowly enough to rescue everyone on the ship.  She just didn’t carry enough lifeboats.  For they thought she was unsinkable.  Because of this lack of lifeboats 1,517 died.  Of course, having enough lifeboats doesn’t guarantee everyone will survive a sinking ship.

The Edmund Fitzgerald was the biggest ore carrier on the Great Lakes during her heyday.  These ships could take an enormous amount of abuse as the storms on the Great Lakes could be treacherous.  Like the one that fell on the Fitzgerald one November night in 1975.  When 30-foot waves hammered her and her sister ship the Arthur Andersen.  No one knows for sure what happened that night but some of the clues indicate she may have bottomed out on an uncharted shoal.  For she lost her handrails indicating that the ship may have hogged (where the bow and stern bends down from the center of the ship held up by that uncharted shoal).  The handrails were steel cables under tension running around the spar deck.  If the ship hogged this would have stretched the cable until it snapped.  She had green water washing across her deck.  Lost both of her radars.  A vent.  Maybe even a hatch cover.  Whatever happened she was taking on water.  A lot of it.  More than her pumps could keep up with.  Causing a list.  And the bow to settle deeper in the water.  Waves crashed over her bow as well as the Andersen’s.  The ships disappeared under the water.  Then reemerged.  As they design ships to do.  Then two massive waves rocked the Andersen.  She was following the Fitzgerald to help her navigate by the Andersen’s radar.  So these two waves had hit the Fitzgerald first.  The Fitzgerald had by this time taken on so much water that she lost too much freeboard.  When she disappeared under these two waves she never came back up.  It happened so fast there was no distress call.  The ship was longer than the lake was deep.  So her screw was still propelling the ship forward when the bow stuck the bottom.  She had lifeboat capacity for all 29 aboard.  But the ship sank too fast to use them.  Or even for the Andersen to see her as she sailed over her as she came to a rest on the bottom.

Our Ships have never been Safer but Ship Owners and Merchants still need to Protect their Wealth with Marine Insurance

We build bigger and bigger ships.  And it’s amazing what can float considering how heavy these ships can be.  But thanks to Archimedes’ principle all we have to do to make the biggest and heaviest ships float is too keep them watertight.  Keeping them less dense than the water that makes them float.  Even if we fail here due to events beyond our control we can isolate the water rushing in by sealing watertight compartments.  And keep them afloat.  So our ships have never been safer.  In addition we have far more detailed charts.  And satellite navigation to carefully guide us to our destination.  Despite all of this ships still sink.  Proving the need for something that has changed little since 14th century Genoa.  Marine insurance.  Because accidents still happen.  And ship owners and merchants still need to protect their wealth.

www.PITHOCRATES.com

Share

Tags: , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ,